1) Field of the Invention
The present invention relates to an art for controlling the output value of impulses when the impulses are generated.
2) Description of the Related Art
Recently, a lot of studies have been made for putting the UWB (Ultra WideBand)-impulse radio communication scheme positively employing an impulse signal (impulse waveform) to practical use.
This UWB-impulse radio communication is a communication scheme in which extremely short impulses are used in place of a carrier wave. Use of the impulses brings about a considerably wide band spectrum, but because of low spectrum density, there is very little interference with other systems.
Two further differences from communication using a carrier are that the UWB-impulse radio communication does not perform power transmission except at the time of impulse transmission, and that it also has an advantage of low power consumption.
When impulses are generated, a circuit employing a step recovery diode (also called a snap-off diode) is used for generating impulses. Under a state where forward voltage is applied to the step recovery diode, reverse voltage is applied thereto, thereby obtaining a voltage waveform with a sharp rise and an extremely high frequency component of several GHz (approximately 20 GHz at maximum). The voltage waveform is then subjected to differentiation and waveform shaping to obtain an impulse.
There has been an art for adjusting the output of impulses generated by the impulse generating circuit by providing a digital control attenuator (digital variable attenuator; for example, an attenuator) at a stage after an impulse generating circuit (for example, see the following patent document 1), and also an art for making the output of an impulse generating circuit constant by feedback control (for example, see the following patent document 2).
The permissive radiation power of UWB radio communication is stipulated so that the peak radiation power and the average radiation power remain below a specified value. For example, according to the FCC (Federal Communications Commission), it is stipulated that the average radiation power of impulses is −41.3 dBm/MHz or lower, and that the peak radiation power of 33.98 dBm/MHz (it is stipulated that +0 dBm/MHz is obtained with a resolution of 50 MHz, and these values are obtained by the conversion formula with a resolution of 1 MHz) within a range of 3.1 GHz and 10.6 GHz. This is called an FCC mask.
Accordingly, in cases where the output value (crest value; hereafter also simply called an output) of impulses is raised for long distance communication, for example, it is necessary to reduce the radiation frequency of impulses to observe the reference value of the mean radiation power. On the other hand, in cases where the radiation frequency is increased to raise the communication speed in short distance communication, it is necessary to reduce the output value of impulses, to observe the reference value of the mean radiation power.
In other words, when the impulse output is increased while the peak value limitation is satisfied, it is possible to lengthen the communication distance by reducing the radiation frequency of impulses. In contrast, when the communication distance is short, by increasing the radiation frequency of impulses, and by decreasing the output of each impulse, high-speed communication becomes available.
The UWB frequency band is also used by other radio communication systems, and it is thus preferable that unnecessarily large-power radiation is avoided to the greatest extent possible for the purpose of avoiding interference.
In this manner, in the UWB radio communication system, the necessity for changing the radiation frequency and the output value of impulses is high, and thus, it is desirable that the mechanism for positively changing the output of impulses is realized by a construction that is as inexpensive and simple as possible.
However, the above-described art needs a digital control attenuator which is relatively expensive. In addition, the digital control attenuator raises the necessity for an interface (or a control circuit) for controlling thereof, and thus manufacturing costs are increased and the construction becomes complicated.
[Patent Document 1] Published Japanese Translation of a PCT Application No. 2003-515974
[Patent Document 2] Published Japanese Translation of a PCT Application No. 2003-529273